Filter units

ABSTRACT

An assay filter unit where the filter of the unit is sealed into the body of the unit to prevent any passage of material to be assayed round the edge of the filter but is removable by breaking out a filter support grid along a predetermined line of weakness, together with the filter, for assay of material retained on the filter.

This invention relates to the preparation of samples for assay wherefiltration and ready access to material retained are required.

In particular the invention addresses the need for many assays to beroutinely, quickly and simply carried out in an inexpensive way andwhere therefore filtration equipment that is complicated and expensiveto make and use will not serve.

With these requirements in mind the invention provides an assay filterunit having single or multiple filter stations and a filter at the oreach station, characterised in that the filter of the or each station issealed into the body of the unit to prevent any passage of material tobe assayed round the edge of the filter but is removable by breaking outtogether with the filtering area of the filter the central portion of afilter support grid lying below the filter, along a predetermined lineof weakness in the grid, for assay of material retained on the filter.

Such units may be used if desired in filtration assemblies comprisingthe assay filter unit and corresponding pre-filter unit(s), stacked forpassage of filtrate from the pre-filter unit(s) to the assay filterunit.

As noted, the assay filter units may be single or in multi-station form,for example a multi-station block used alone or with separate pre-filterunits.

Conveniently the line of weakness is provided by notches of vee or otherstress-concentrating form (as seen insection axially of the filterstation) formed in elements of the grid at its periphery.

For ease of production and use it is preferred that the body of theassay unit and the grid are integrally formed of plastics by injectionmoulding and the axial thickness of the elements of the grid at thenotches is 0.15 to 0.5 mm, desirably 0.2 to 0.3 mm, thereby allowingboth for ready flow of material during moulding and for ready breakingout of the grid after use.

Conveniently the assay filter unit, of plastics, has a plastics ringclamping the filter against the periphery o the support grid, welded(particularly ultrasonically welded) in place. In this construction itis desirable that the assay filter unit body and the ring where theyabut the filter have a surface roughness of up to 0.5 microns to improvegrip on the filter. Such roughness can be given by controlled sandblasting of the appropriate parts of injection moulds in which the unitsand rings are to be produced, and the grip it gives leads to the filterbreaking out cleanly with the grid.

The invention further provides, for use with the single filter stationunits, a suction manifold having a plurality of stations for receptionof a corresponding number of units and an anvil over which,successively, the body of each unit can be placed to break out thesupport grid.

In a particular application the invention may provide filtrationassemblies as above for carrying out a microbiological assay fordetecting a predetermined microorganism or cell in a sample containingparticles larger than the predetermined microorganism or cell, eachassembly including at least one first filter unit (pre-filter unit)including an open container with a filter having pores large enough topermit the passage therethrough of the predetermined microorganism orcell and small enough to prevent the passage therethrough of theparticles larger than the predetermined microorganism or cell, and atleast one second filter unit (assay filter unit), verticallyinterstackable with its respective first such unit, including an opencontainer with a filter having pores large enough to permit the passagetherethrough of aqueous liquid and small enough to retain thepredetermined microorganism or cell, the filter of the or each secondfilter unit being, as before, sealed into the unit by a retaining ringor other means preventing any passage of material round the edge of thefilter but removable from the unit by breaking out, along apredetermined line of weakness, a filter support lying below the filter,in order to conduct assay.

In such an application as many successively smaller-pore pre-filterunits can be stacked as are needed to remove particulates from aparticular sample without clogging the filters. For example, theassembly may include one or more third filter units (additionalpre-filter units), one on top of each first filter unit, each thirdfilter unit including a filter having pores larger than the pores of thefilter of the succeeding pre-filter unit and small enough to prevent thepassage therethrough of a portion of the particles in the sample largerthan the predetermined microorganism or cell.

The area of the filter of pre-filter units where used should be largeenough that the likelihood of clogging by the particles to be retainedis small. A convenient area is 20 mm² to 500 mm². The area of the filterof the or each assay filter unit should correspondingly be large enoughto prevent clogging by particles, in any reasonable sample,approximating the size of or smaller than the predeterminedmicroorganism or cell, but small enough to concentrate the predeterminedmicroorganism or cell conveniently for assay.

Usually, the units are cylindrical but in other embodiments, forexample, the pre-filter unit is conical, its filter matching the size ofthe filter of the corresponding assay filter unit, allowing largersamples to be handled readily.

Other convenient optional features are that pre- filter units where usedare of different colours to the assay filter units and that the assayfilter units are transparent, with a fill line indicating amounts ofreagents to be added in course of the assay.

Suitably the units are injection moulded, for which materials such aspolypropylene, polystyrene, acrylic or modified acrylic orpolyvinylchloride plastics are suitable. Filters of any convenientmaterial and pore size can be used in the units, made for example fromcellulose acetate, nylon, or nitrocellulose. The size and shape of theunits may of course be varied to meet the requirements of varying samplecharacteristics, including concentration and size of particulate matter,and concentration and size of the microorganism or cell of interest.

In a particular embodiment the assay filter unit has a recessed basebelow the filter, and is used with a reversible cap having a firstportion and a second portion, the perimeter of the first portion beingsmaller than the perimeter of the second portion, the second portionbeing adapted to fit snugly on top of the unit or, optionally, anypre-filter unit and the first portion being adapted to fit snugly intothe recessed base to provide a leak-proof closure.

For use with microorganisms, the filter of the assay filter unitconveniently has pores of 0.02 to 3 microns diameter; the filter of anypre-filter unit then has pores of a diameter 1 to 50 microns.

The units may for example be adapted to test a diluted sample having avolume of 1 ml upwards, e.g. to 50 ml. according to the number of units.Small volumes apply for example to a standard 96 well unit, largervolumes where fewer stations are involved.

In preferred form the units of the invention can provide many advantagesseparately or together. Thus an assembly of assay filter and pre-filterunits can permit the removal of large particulate matter during the samevacuum filtration step in which the microorganisms or cells of interestare deposited on the filter, the assembly eliminating the need fortransfer of the microorganisms or cells before processing of the samplecan begin. All of the filter units can be disposable, eliminating theneed for the washing and sterilizing steps required for example inreusable hybridization assay apparatus. Snug fit between units can beprovided, preventing leakage or accidental disassembly during use.Colour coding of units can facilitate assembly and prevent accidentalreversal of like-sized filter units. A suitably rigid grid on the second(assay) filter unit, and the ring above the filter, can provide supportfor a fragile filter, protecting it from deformation during suction. Afill line on a transparent assay filter unit can permit easy addition ofsolution from, e.g., a squeeze applicator bottle, obviatingtime-consuming measurement of reagents. A reversible cap can be placedon any same-sized filter unit to prevent drying and contamination of thesample with air-borne contaminants. The same cap if suitably designedcan be reversed and used as a base for a suitably dimensional assayfilter unit to prevent leakage during steps of the assay carried outbefore breaking out the filter. A manifold, or if desired a manifoldcover applied to the manifold itself only when ready for suction, canhold a number of units containing multiple and/or duplicate samples,which can all be processed at the same time. Such a manifold cover analso serve as a convenient tray for holding the units steady while theyare being loaded with samples, and for carrying them about.

Uses in particular include assay for bacteria of the genus Salmonellabut many other uses applicable to food samples, to human or veterinarybody fluid or other samples, and to other materials exist.

Other features and advantages of the invention will be apparent from thefollowing description of particular embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view, partially broken away, of the parts of aparticular assembly, with reversible cap;

FIG. 2 is an isometric view, partially broken away, of the assay filterunit of said assembly, which may of course also be used alone;

FIG. 3 is a part-sectional elevation of the body of a further assayfilter unit;

FIG. 4 is an underneath plan view of said body showing at A--A thesection line of FIG. 3;

FIG. 5 is a sectional view of a suction manifold;

FIG. 6 is a view of a suction manifold cover;

FIG. 7 is a sectional elevation on B-B of FIG. 6 showing part of asuction manifold and, in outline, a filtration assembly in place.

Referring to FIG. 1, a multiple-unit assembly includes first (pre)filter unit 12, second (assay) filter unit 14, and reversible cap 16,including top portion 18 and bottom portion 20.

The container of unit 12 is of blue injection-molded polypropylene, andthe container of unit 14 is transparent polypropylene. Container 14bears circumferential fill line 15.

Each of units 12 and 14 has a rim portion, 22 and 24, which fits intothe recessed portion 26 of the unit above it, or into bottom portion 20of cap 16. Top portion 18 of cap 16 alternatively fits snugly intorecessed portion 28 of unit 14. Each unit is approximately 2.5 cm highfrom recess to rim, and has a volume of about 50 ml.

As is best shown in FIG. 2, each filter unit (designated 1) includessupport 30, filter 32, and filter hold-down ring 34. Support 30 isinjection moulded integral with unit 1 and has a circumferential bordersurrounding a grid. The border is wide enough to provide a continuoussurface against which the top surface of a rim of a lower filter unitcan abut without being so wide as to interfere with the passage of thefiltrate. The grid may be of any open pattern so long as the overlyingfilter is supported and sufficient space is allowed for passage of thefiltrate but the preferred form is that of FIG. 4.

Filter 32, overlying support 30 and held down by ring 34, is the primaryfunctional portion of the unit. The pore size of filter 32 depends onthe unit's purpose. The filter of unit 12 (FIG. 1) has for example poressufficiently large to allow a microorganism of interest (e.g.Salmonella) to pass through but sufficiently small to retain largerbacteria and particulates. The filter 32 of unit 14 is a filter membranewhich retains the microorganism and which is suitable for use in theassay.

Filter 32, as mentioned above, is secured within unit 1 by means of ring34, which is thermally or ultrasonically welded into the unit to form acomplete seal.

The details of the grid are seen in FIGS. 3 and 4. It is approximatelyone and one quarter mm in thickness axially of the unit with a 60°included-angle vee notch 40 moulded into each element 42 of it, leavinga web thickness, at 41 and as measured axially of the unit, of onequarter mm. This web allows for flow of material during moulding and isboth strong enough to withstand handling and filtration suctionpressures and weak enough to break out easily against an anvil undermanual pressure only. The upper faces of the grid elements 42 arerounded to support filter 32 without risk of rupture and also toincrease the effective free filter area. The form of the grid is seen inFIG. 4 and is designed for maximum support with minimum interferencewith flow, especially important when small pore size filters are in use.

The manifold of FIG. 5 is designed to receive filtration assemblies, orassay filter units alone, specifically those of FIGS. 3 and 4 at fivestations 51. The base portion of the body of a given filter unit fitsinto one of the annular recesses 52 of the manifold, with annular land53 contacting the unit within the base of the unit at 44. Suctionapplied through bore 54 then draws filtrate from a sample placed in a(or an uppermost) pre-filter unit into the assay filter unit and thenthrough the assay filter to waste, leaving the organism or othermaterial to be assayed on the filter of the assay unit. Unused stations,or assemblies or units filtering faster than others, can be covered withcaps 16 if required. When filtration is complete, (and it will beappreciated that according to the design of the assemblies and theapplication, it may be by centrifugation rather than suction), anypre-filter units are discarded and any washing or reagent treatmentsrequired using filtration may be carried out. Then the assay filterunits are transferred in turn to anvil station 55 where they fit inannular recess 56. Anvil 57 is sharp edged and dimensioned to press outthe grid and overlying filter, breaking the grid out at the apex ofnotches 40 and passing internally of ring 34. Only hand pressure isrequired and the filter may be dropped directly from the body of theunit into a receptacle for further treatment. The unit itself and thebroken grid are discarded.

In a particular application, the filtration assembly is used to preparefood samples for a nucleic acid hybridization assay for Salmonellabacteria

For example, filter 32 (FIG. 2) of filter unit 12 (FIG. 1) is amicroporous polymeric membrane having pores of nominal diameter 1 to 50microns. These pore sizes are large enough to allow the passagetherethrough of Salmonella bacteria and small enough to retain largerbacteria and large food particles The diameter of this filter isapproximately 2.4 cm, providing an area large enough to prevent cloggingby particles of a typical food sample diluted 10:1 with water.

The filter of unit 14 has pores of 0.2 to 3 microns, small enough toretain Salmonella bacteria and large enough to allow the passage ofaqueous liquid. The diameter of the filter is 2.4 cm, providing an arealarge enough to prevent clogging by the Salmonella-sized and smallerparticles which are present in some food samples (e.g. flour and peanutbutter) and small enough to provide a high enough concentration ofSalmonella bacteria on at least a portion of the filter to render thehybridization assay straightforward.

Referring now to FIGS. 6 and 7, a manifold cover 64 of injection-moldedpolystyrene, contains a plurality of cylindrical wells 66. Lip 68 ineach of wells 66 prevents the units from extending through manifoldcover 64. The cover fits over a manifold base of cast epoxy (FIG. 7),which fits a conventional vacuum-suction device (not shown).

A sample of food to be tested for Salmonella is diluted 10 fold withwater and poured into unit 12 (FIG. 1); the assembly is at this pointassembled and resting on one of the wells of manifold cover 64. Vacuumis applied, causing liquid and small particles, including Salmonella, topass through the filter of unit 12, while larger bacteria and foodparticles are retained Unit 12 is then discarded.

Prehybridization buffer (of conventional formula) is then added to unit14, up to fill line 15. Vacuum is again applied. Hybridization buffer,including a labelled probe is then added and unit 14 is removed from themanifold, cap 16 is inserted in recess 28, and the unit is incubated at37° for 2 hours. The filter support is then broken out and the filterremoved, and labelled complexes are then detected as an indication inthe sample of Salmonella bacteria.

I claim:
 1. An assay filter unit for filtering a sample and comprising,in combination:a body having an internal passageway: a filter disposedwithin said passageway; a filter support means for supporting saidfilter and seal within said body so as to prevent passage of any of saidsample around the edges of said filter, and provided with a line ofmechanical weakness disposed peripherally about said support means sothat said filter is removable from said body by breaking said supportmeans along said line.
 2. A filter unit according to claim 1 whereinsaid filter support means is a grid and said line of weakness is formed,at least in part, by notches of stress-concentrating form in elements ofsaid grid.
 3. A filter unit according to claim 2 wherein said body andsaid grid are integrally molded together of synthetic polymer.
 4. Afilter unit according to claim 3 wherein the axial thickness of saidgrid at said notches is predetermined to allow for both ready flow ofmaterial during molding and for ready breaking out of the grid afteruse.
 5. A filter unit according to claim 4 wherein said axial thicknessat said notches is in the range of about 0.15 to 0.5 mm.
 6. A filterunit according to claim 4 wherein said axial thickness at said notchesis in the range of about 0.2 to 0.3 mm.
 7. A filter unit according toclaim 1 wherein said unit is formed of synthetic polymer and includes aring clamping said filter against the periphery of said support grid andwelded in place.
 8. A filter unit according to claim 7 wherein said bodyand said ring, where they abut said filter, have surfaces of roughnesssufficient to improve the grip on said filter.
 9. A filter unitaccording to claim 1 wherein disposed within a suction manifold having aplurality of stations for receiving a corresponding number of similarunits, and an anvil over which, successively, said body of each saidunit can be placed to break out the corresponding said grid along thecorresponding line of weakness.
 10. A filtration assembly, comprising,in combination, a plurality of assay units, each comprising:a bodyhaving an internal passageway: a filter disposed within said passageway;a filter support means for supporting said filter and seal within saidbody so as to prevent passage of any of said sample around the edges ofsaid filter, and provided with a line of mechanical weakness disposedperipherally about said support means so that said filter is removablefrom said body by breaking said support means along said line; saidunits being stacked for passage of filtrate therethrough from one tosuccessive units.